The Fabry-Perot (FP) type including an optical resonator flanked by mirrors provided on both end faces of an active layer has been generally utilized for a semiconductor laser. However, according to this FP type laser, light is oscillated at a wavelength satisfying conditions for oscillating standing waves. The longitudinal mode thus tends to be of multi-mode, and the oscillation wavelength is changed particularly when a current or temperature is changed, resulting in a change of optical intensity.
Therefore, for the purpose of optical communication, gas sensing and so forth, required is a laser capable of a single mode oscillation with high wavelength stability. For this reason, a distributed feed-back type (DFB) laser and a distributed Bragg reflection type (DBR) laser have been developed. Such a laser, a diffraction grating is provided in a semiconductor to oscillate only light of a specific wavelength by utilizing wavelength dependency thereof.
In order to realize a semiconductor laser that exhibits wavelength stability, the DBR laser and the DFR laser each in which gratings are formed monolithically in the semiconductor laser, and an external resonator type laser in which a fiber Bragg grating (FBG) is fixed as a grating to the outside of a laser can be exemplified. These are based on the principle by which a part of laser light is fed back to the laser with a wavelength selective mirror utilizing Bragg reflection to realize an operation for wavelength stability.
According to the DBR laser, concaves and convexes are formed on the surface of a waveguide extended from the waveguide of the active layer to configure a mirror by utilizing Bragg reflection for realizing a resonator (Patent document 1; JP S49-128689 A: Patent document 2; JP 856-148880 A). Since diffraction gratings are provided on both ends of an optical waveguide layer according to this laser, light emitted from the active layer is propagated through the optical waveguide layer; a part of the propagating light is reflected by the diffraction gratings and returned into a current injection part; and then amplified. As light having only a specific wavelength is reflected in the determined direction from the diffraction grating, the wavelength of the laser light is made constant.
Further, as the application, an external resonator type semiconductor laser, for which diffraction gratings are used as parts different from the semiconductor, and an external resonator is formed, has been developed. This type of laser provides lasers each exhibiting excellent wavelength stability, temperature stability and controllability. The external resonator includes a fiber Bragg grating (FBG) (Non-patent document 1) and a volume holographic grating (VHG) (Non-patent document 2). Since the diffraction grating is composed of a member different from a semiconductor laser, it is a feature that the reflectance as well as the length of the resonator can be independently designed, and it is not affected by the influence of a rise in temperature caused by heat generation via current injection, and thus the wavelength stability can be further improved. Further, the semiconductor exhibits different temperature change of refractive index therefrom, and thus the temperature stability can be improved by designing the foregoing together with the length of the resonator.
An external resonator type laser, in which gratings formed in a quartz glass waveguide are utilized, has been disclosed in Patent document 6 (JP 2002-134833 A). This aims at providing a frequency stabilized laser which can be used in an environment under which the room temperature is largely changed with no temperature controller (for example, a temperature of 30° C. or higher). It is also described that mode hopping is suppressed, and a temperature independent laser exhibiting no temperature dependency on the oscillation frequency is provided.
Further, it has been proposed that a plurality of Bragg gratings is formed in an optical fiber to stabilize a wavelength (Patent document 7).
An external resonator type light-emitting device, provided with a light source by which semiconductor laser light is oscillated and a plurality of Bragg gratings each having a different period, has been disclosed in Patent document 8. The reflection characteristics of the Bragg gratings each are independent and are not crossed with each other, thereby possessing no reflection characteristics having a continuously broad wavelength. Further, the present device provides the concept such that gratings and a phase adjusting region have been formed on the light source side, and the wavelength is varied by the action of this phase adjusting region to realize excellent wavelength stability even upon temperature change.    [Non-patent document 1] Transactions on Fundamentals of Electronics, Communications and Computer Sciences C-II Vol. J81, No. 7, pp. 664-665, July 1998    [Non-patent document 2] Technical Reports on Fundamentals of Electronics, Communications and Computer Sciences LQE, Vol. 105, No. 52, pp. 17-20, 2005    [Non-patent document 3] Furukawa Review No. 105, pp. 24-29, January 2000    [Patent document 1] JP S49-128689 A    [Patent document 2] JP S56-148880 A    [Patent document 3] WO 2013/034813 A    [Patent document 4] JP 2000-082864 A    [Patent document 5] JP 2006-222399 A    [Patent document 6] JP 2002-134833 A    [Patent document 7] JP 2002-006148 A    [Patent document 8] US 2003/0108081 A1